1. Field of the Invention
The present invention generally relates to vertical SCR-type switches. “SCR-type switches” is used to designate components such as thyristors, triacs, and other controlled bi-directional switches comprising at least four or five semiconductor layers of alternated doping types.
2. Discussion of the Related Art
FIG. 1 is a cross-section view of a conventional thyristor structure. The thyristor comprises an N-type cathode region 1, formed in a P-type well 2, itself formed in a lightly-doped N-type substrate 3, and a P-type anode region 4 on the rear surface side. A thyristor in which substrate 3 is surrounded with a P-type insulating wall 5 in contact with anode 4 has been shown. P-type well 2 corresponds to a cathode-gate area. The front surface of the thyristor is covered with an insulating layer 6. A cathode metallization MK connected to a cathode terminal K is in contact with cathode region 1. A gate metallization MG connected to a gate terminal G is in contact with well 2. The rear surface of the component is coated with an anode metallization MA connected to an anode terminal A.
Cathode region 1 as well as other portions of well 2, of substrate 3, and of anode region 4 located underneath form a power area through which a significant current flows when the thyristor is activated. The thyristor elements located above and close to gate metallization MG form a control area which is active upon turning-on of the thyristor.
FIG. 2 is a diagram of an HF control circuit 10 of a thyristor 11 inserted in a power circuit 12, such as described in U.S. patent application Ser. No. 10/727,189, filed Dec. 3, 2003, which is incorporated herein by reference. Power circuit 12 is schematically shown as a voltage generator V1 and a load L in series between the anode and the cathode of thyristor 11. Control circuit 10 is intended to apply a current between gate G and cathode K of the thyristor and essentially comprises in series a sinusoidal high-frequency HF voltage generator and a switch Sw. The above-mentioned patent application explains that, even though the power provided by the control circuit during each period is smaller than the power necessary to turn on the thyristor, said thyristor surprisingly turns on after a number of periods.
When the power provided by the control circuit is much smaller than the power required to turn on the thyristor, the switch activation time may however be relatively long.
Further, the thickness and the doping profile of the elements of the thyristor of FIG. 1 are most often optimized to improve the thyristor conductivity as well as its breakdown voltage, which often goes against an optimization of the switch starting. The structural features of the thyristor are thus not favorable to the reduction of the activation time.
The control of a thyristor with a high-frequency signal enables providing a galvanic isolation between the HF voltage generator and the thyristor by placing a first capacitor between switch Sw and gate G of the thyristor and a second capacitor between the HF generator and cathode K of the thyristor. The first capacitor can be integrated in the thyristor by providing an insulating layer between the gate metallization and the gate semiconductor area. The second capacitor however cannot be integrated in similar fashion since a capacitor cannot be placed on a “power” path conducting strong D.C. currents or currents at the low mains frequency. A complete galvanic isolation thus requires providing a discrete capacitor between the HF generator and the thyristor cathode. Now, generally, it is rather desired to decrease the number of discrete components for reasons of cost, space, and reliability.